Silver-containing direct thermographic imaging materials are non-photosensitive materials that are used in a recording process wherein images are generated by the direct application of thermal energy and in the absence of a processing solvent. These materials have been known in the art for many years and generally comprise a support having disposed thereon one or more imaging layers comprising (a) a relatively or completely non-photosensitive source of reducible silver ions, (b) a reducing agent composition (acting as a black-and-white silver developer) for the reducible silver ions, and (c) a suitable hydrophilic or hydrophobic binder. Thermographic materials are sometimes called “direct thermal” materials in the art because they are directly imaged by a source of thermal energy without any transfer of the image, or image-forming reactants to another material (such as in thermal dye transfer).
In a typical thermographic construction, the image-forming thermographic layers comprise silver salts of long chain fatty acids. The preferred non-photosensitive reducible silver source is a silver salt of a long chain aliphatic carboxylic acid having from 10 to 30 carbon atoms, such as behenic acid or mixtures of acids of similar molecular weight. At elevated temperatures, the silver(I) of the silver carboxylate is reduced by a black-and-white reducing agent (also known as a developer) whereby a black-and-white image of elemental silver is formed.
Problem to be Solved
As noted above, direct thermographic materials are imaged by a recording process whereby images are generated by imagewise heating a recording material containing chemical components that provide an optical density change in an imagewise fashion. A major consideration in designing such materials is to find a combination of imaging chemicals that provide optimum image maximum density Dmax without loss of other properties. A further consideration is to find a combination of imaging chemicals that provide increased “silver efficiency” (that is, a high Dmax using only a small amount of silver).
Thus, there remains a continuing need to incorporate compounds into black-and-white direct thermographic materials to provide materials that generate a dense black metallic silver image and have high silver efficiency.